So confused on adjusted age works for preemie.

Why is it that the medically wise, they go by their birthdate.. If my baby is born at 33 weeks and today he’s 40 weeks. At 41 weeks they expect him to get his 2 months vaccines cause he was born 8 weeks ago but then again he is technically only 1 week old if he would’ve been born on his due date at 40 weeks. So do we consider him to be out of the newborn phase (immune wise) sooner than if we were counting off his due date?